Composite planetary devices comprised of a planetary gear mechanism and planetary roller mechanism are disclosed by JP A 9-168910 and JP A 2002-213566, for example. As disclosed in these patent documents, a planetary gear speed reducer can transmit a large torque but has the problems of backlash and meshing noise. A planetary roller speed reducer has zero backlash and low noise. However, it continually produces slippage, which makes it unsuitable for positioning mechanisms and the like, and also has the problem of not being able to transmit a large torque. A composite planetary device can mutually compensate for the drawbacks of a planetary gear speed reducer and planetary roller speed reducer, making it possible to effectively utilize the merits of both mechanisms.
However, conventional composite planetary devices have the following problems. With the composite planetary device disclosed by the above JP A 9-168910 and the like, as shown in the schematic diagram of FIG. 4, the radius of the working pitch circle of each of the gears of the planetary gear mechanism and the radius of each of the rollers of the planetary roller mechanism are the same. Therefore, the speed reduction ratio Ug of the planetary gear mechanism and the speed reduction ratio Ur of the planetary roller mechanism are the same, and the slippage ratio s1 between the sun roller and planetary roller and the slippage ratio S2 between the planetary roller and the ring roller are both zero. In this case, the roller drive force (tractive force) of the planetary roller mechanism is small, and the output torque is small.
As disclosed in JP A 2002-213566, in order to increase the torque, the roller drive force of the planetary roller mechanism can be increased by generating slippage between the rollers of the planetary roller mechanism. FIG. 5 is a schematic diagram of this case. Here, the radius of the sun roller is made larger by Δr1 than the radius r1 of the working pitch circle of the sun gear, along with which the radius of the planetary roller is made smaller by Δr1 than the radius r2 of the working pitch circle of the planetary gear, and the internal radius of the ring roller is also made smaller by Δr1 than the radius r3 of the working pitch circle of the internal gear. In this case, the speed reduction ratio Ur of the planetary roller mechanism is different from the speed reduction ratio Ug of the planetary gear mechanism, and slippage is generated between the sun roller and the planetary roller, and between the planetary roller and the ring roller.
In this case, the slippage between the planetary roller and the ring roller is in the opposite direction from the slippage between the sun roller and the planetary roller. For example, if a positive slippage ratio s1 is applied between the sun roller and the planetary roller, a negative slippage ratio s2 is generated between the planetary roller and the ring roller. Thus, the roller drive forces generated between the rollers cancel each other out, making it impossible to obtain a large output torque.